The invention relates to a liquid crystal display device provided with a first electrode and a second electrode for applying the voltage to a liquid crystal layer.
In case of erasing images displayed on a liquid crystal display by means of turning off the power supplied to the concerned display, there are some liquid crystal displays in which the time between the moment at which the power supplied to the said liquid crystal display has been turned off and the full erasure of the image from said liquid crystal display (said time will be referred to as xe2x80x9cerasing timexe2x80x9d hereinafter) is needed 4 to 5 seconds or even about 30 seconds. The reason of the longer erasing time may exist mainly in that the voltage having a certain magnitude may be still applied to a liquid crystal layer for a while even after the turnoff of the power supply. The longer erasing time results in that the afterimage remains on the display for the longer time. Since such afterimage is obtrusive to the user, it is required to shorten the erasing time in such a way that the afterimage erases as quickly as possible.
One of the known techniques for shortening the erasing time in case of, for example, TFT type liquid crystal display devices, is a method for providing a gate driver with a function of switching all TFTs to the ON state immediately after the power for the liquid crystal display device has been turned off (such function will be referred to as xe2x80x9cALL-ONxe2x80x9d function hereinafter). If a gate driver provided with such function is used, the OFF image data could be written to pixel electrodes immediately after the power for the liquid crystal display device has been turned off, so that the potential of the pixel electrodes may be immediately changed to a zero potential. Accordingly, the erasing time can be shortened because the potential difference between the pixel electrodes and the common electrode becomes substantially zero in a short time.
In the case of performing the ALL-ON function of the gate driver, a power detection circuit or a signal detection circuit which are dedicated for performing the ALL-ON function is additionally required. The power detection circuit detects the externally supplied voltage and controls the ALL-ON function in accordance with the detected voltage. The signal detection circuit detects not only the externally supplied voltage but also a signal (for example, horizontal synchronization signal) or detects only said signal and controls the ALL-ON function in accordance with the detected voltage and signal or only said signal.
In the case of using such voltage detection circuit, there is a problem of increasing the cost because an expensive voltage detection IC is required. On the other hand, in the case of using the signal detection circuit, there is also a problem that the specification of the signal detection circuit must be changed depending on the characteristic (e.g., amplitude and/or frequency) of the signal to be detected.
From a viewpoint of the aforementioned situation, it is an object of the invention to provide a liquid crystal display device that is less expensive but capable of shortening the erasing time without detecting, for example, the horizontal synchronization signal.
A first liquid crystal display device in accordance with the invention in order to achieve the above-described objective comprises a first electrode and a second electrode for applying a voltage to a liquid crystal layer, a first bus and a second bus that are electrically connected to said first electrode via first switching means, potential generation means for generating a first potential that is supplied toward said first switching means via a path containing said first bus, a charge flowing portion into which electric charges existing in said path, said first electrode or said potential generation means may flow and a second switching means for switching a state of the flow of electric charges into said charge flowing portion to either a first sate in which said electric charges flow into said charge flowing portion or a second state in which said electric charges do not flow into said charge flowing portion so much as in said first state.
The first liquid crystal display device in accordance with the invention is provided with the charge flowing portion into which electric charges existing in said path, said first electrode or said potential generation means may flow. Furthermore, the state of the flow of electric charges into this charge flowing portion is switched by the second switching means. Accordingly, when this charge flowing portion is shifted from the second sate to the first state, the electric charge existing in said path, said first electrode or said potential generation means could efficiently flow into this charge flowing portion, and as a result, the potentials of said path, said first electrode or said potential generation means could be quickly changed by an potential corresponding to the amount of electric charges that have flowed into this charge flowing portion. Thus, the erasing time could be shortened, as will be later described, by means of changing the potentials of said path, said first electrode or said potential generation means. Besides, with the aforementioned charge flowing portion, it is possible to shorten the erasing time at a low cost without detecting, for example, the horizontal synchronization signal as will be described later.
In accordance with a first aspect of the invention, it is preferable that said charge flowing portion is set to said first state when said second switching means is in an ON state whereas said charge flowing portion is set to said second state when said second switching means is in an OFF state. Thus, the charge flowing portion could be set to either first state or second state by means of switching said second switching means to either ON or OFF state.
In accordance with a second aspect of the invention, the aforementioned first liquid crystal display device preferably further comprises control means for controlling said second switching means so that said second switch means is switched to either an ON state or an OFF state. With such control portion, the switching between the ON state and the OFF state of said second switching means could be easily performed.
In accordance with a third aspect of the invention, said potential generation means for the aforementioned first liquid crystal display device generates a plurality of potentials, and that said control portion detests said plurality of potentials generated by said potential generation means and controls said second switching means so that said second switch means is switched to either an ON state or an OFF state on the basis of said detected potentials. In accordance with such structure of the control portion, the control portion does not need to detect a signal (for example, horizontal synchronization signal), and as a result, the control portion could be designed without reference to the signal characteristic.
In accordance with a fourth aspect of the invention, the aforementioned first liquid crystal display device preferably further comprises a first driver for transmitting signals to said first bus and a second driver for transmitting signals to said second bus, and that said potential generation means generates a second potential to be supplied toward said first driver and a third potential to be supplied toward said second driver in addition to said first potential, and that said control portion detects said first, second and third potentials and controls said second switching means so that said second switching means is switched to either an ON state or an OFF state on the basis of said detected potentials. By means of detecting these first, second and third potentials generated by said potential generation means, the control portion could be designed without reference to the signal characteristic.
In accordance with a fifth aspect of the invention, said control portion for the aforementioned first liquid crystal display device preferably comprises a third switching means for switching an ON state and an OFF state of said second switching means. Through easy switching of said third switching means, the switching between the ON state and the OFF state of said second switching means could be easily controlled.
Furthermore, in the aforementioned first liquid crystal display device, said first electrode may be a pixel electrode and said second electrode may be a common electrode, said first bus may be a gate bus and said second bus may be a source bus, and said first driver may be a gate driver and said second driver may be a source driver.
Moreover, the invention provides a second liquid crystal display device comprising a first electrode and a second electrode for applying a voltage to a liquid crystal layer, a first bus and a second bus which are electrically connected to said first electrode via first switching means, and potential generation means for generating a first potential which is supplied toward said first bus, characterized in that said potential generation means generates a second potential to be supplied toward said first bus when the supply of the power for said potential generation means has been stopped, said second potential being larger than said first potential.
In particular, the potential generation means provided in the aforementioned second liquid crystal display device generates the second potential larger than said first portion when the supply of the power for said potential generation means has been stopped. That second potential is supplied toward said first bus. By means of the supply of the second potential larger than the first potential toward the first bus when the supply of the power for said potential generation means has been stopped, the erasing time could be shortened as will be later described. Besides, in accordance with the aforementioned potential generation means provided in the second liquid crystal display device, it is possible to shorten the erasing time at a low cost without detecting, for example, the horizontal synchronization signal as will be described later.
In accordance with a further aspect of the invention, said potential generation means in the aforementioned second liquid crystal display device preferably comprises a differential amplifier that outputs said second potential. With such differential amplifier, the second potential could be generated through a simple circuit structure.
Furthermore, in the aforementioned second liquid crystal display device, said first electrode may be a pixel electrode and said second electrode may be a common electrode, and said first bus may be a gate bus and said second bus may be a source bus.